Haskell defines operations to read and write characters from and to files,
represented by values of type Handle. Each value of this type is a
handle: a record used by the Haskell run-time system to manage I/O
with file system objects. A handle has at least the following properties:

whether it manages input or output or both;

whether it is open, closed or semi-closed;

whether the object is seekable;

whether buffering is disabled, or enabled on a line or block basis;

a buffer (whose length may be zero).

Most handles will also have a current I/O position indicating where the next
input or output operation will occur. A handle is readable if it
manages only input or both input and output; likewise, it is writable if
it manages only output or both input and output. A handle is open when
first allocated.
Once it is closed it can no longer be used for either input or output,
though an implementation cannot re-use its storage while references
remain to it. Handles are in the Show and Eq classes. The string
produced by showing a handle is system dependent; it should include
enough information to identify the handle for debugging. A handle is
equal according to == only to itself; no attempt
is made to compare the internal state of different handles for equality.

GHC note: a Handle will be automatically closed when the garbage
collector detects that it has become unreferenced by the program.
However, relying on this behaviour is not generally recommended:
the garbage collector is unpredictable. If possible, use explicit
an explicit hClose to close Handles when they are no longer
required. GHC does not currently attempt to free up file
descriptors when they have run out, it is your responsibility to
ensure that this doesn't happen.

Three kinds of buffering are supported: line-buffering,
block-buffering or no-buffering. These modes have the following
effects. For output, items are written out, or flushed,
from the internal buffer according to the buffer mode:

line-buffering: the entire output buffer is flushed
whenever a newline is output, the buffer overflows,
a System.IO.hFlush is issued, or the handle is closed.

block-buffering: the entire buffer is written out whenever it
overflows, a System.IO.hFlush is issued, or the handle is closed.

no-buffering: output is written immediately, and never stored
in the buffer.

An implementation is free to flush the buffer more frequently,
but not less frequently, than specified above.
The output buffer is emptied as soon as it has been written out.

Similarly, input occurs according to the buffer mode for the handle:

line-buffering: when the buffer for the handle is not empty,
the next item is obtained from the buffer; otherwise, when the
buffer is empty, characters up to and including the next newline
character are read into the buffer. No characters are available
until the newline character is available or the buffer is full.

block-buffering: when the buffer for the handle becomes empty,
the next block of data is read into the buffer.

no-buffering: the next input item is read and returned.
The System.IO.hLookAhead operation implies that even a no-buffered
handle may require a one-character buffer.

The default buffering mode when a handle is opened is
implementation-dependent and may depend on the file system object
which is attached to that handle.
For most implementations, physical files will normally be block-buffered
and terminals will normally be line-buffered.

For a readable handle hdl, hIsEOFhdl returns
True if no further input can be taken from hdl or for a
physical file, if the current I/O position is equal to the length of
the file. Otherwise, it returns False.

NOTE: hIsEOF may block, because it has to attempt to read from
the stream to determine whether there is any more data to be read.

The action hSetEncodinghdlencoding changes the text encoding
for the handle hdl to encoding. The default encoding when a Handle is
created is localeEncoding, namely the default encoding for the current
locale.

To create a Handle with no encoding at all, use openBinaryFile. To
stop further encoding or decoding on an existing Handle, use
hSetBinaryMode.

hSetEncoding may need to flush buffered data in order to change
the encoding.

Note that the TextEncoding remembers nothing about the state of
the encoder/decoder in use on this Handle. For example, if the
encoding in use is UTF-16, then using hGetEncoding and
hSetEncoding to save and restore the encoding may result in an
extra byte-order-mark being written to the file.

The action hFlushAllhdl flushes all buffered data in hdl,
including any buffered read data. Buffered read data is flushed
by seeking the file position back to the point before the bufferred
data was read, and hence only works if hdl is seekable (see
hIsSeekable).

This operation may fail with:

isFullError if the device is full;

isPermissionError if a system resource limit would be exceeded.
It is unspecified whether the characters in the buffer are discarded
or retained under these circumstances;

isIllegalOperation if hdl has buffered read data, and is not
seekable.

Returns a duplicate of the original handle, with its own buffer.
The two Handles will share a file pointer, however. The original
handle's buffer is flushed, including discarding any input data,
before the handle is duplicated.

Computation hClosehdl makes handle hdl closed. Before the
computation finishes, if hdl is writable its buffer is flushed as
for hFlush.
Performing hClose on a handle that has already been closed has no effect;
doing so is not an error. All other operations on a closed handle will fail.
If hClose fails for any reason, any further operations (apart from
hClose) on the handle will still fail as if hdl had been successfully
closed.

Computation hSeekhdl mode i sets the position of handle
hdl depending on mode.
The offset i is given in terms of 8-bit bytes.

If hdl is block- or line-buffered, then seeking to a position which is not
in the current buffer will first cause any items in the output buffer to be
written to the device, and then cause the input buffer to be discarded.
Some handles may not be seekable (see hIsSeekable), or only support a
subset of the possible positioning operations (for instance, it may only
be possible to seek to the end of a tape, or to a positive offset from
the beginning or current position).
It is not possible to set a negative I/O position, or for
a physical file, an I/O position beyond the current end-of-file.

Specifies the translation, if any, of newline characters between
internal Strings and the external file or stream. Haskell Strings
are assumed to represent newlines with the '\n' character; the
newline mode specifies how to translate '\n' on output, and what to
translate into '\n' on input.

Map '\r\n' into '\n' on input, and '\n' to the native newline
represetnation on output. This mode can be used on any platform, and
works with text files using any newline convention. The downside is
that readFile >>= writeFile might yield a different file.

NOTE for GHC users: unless you use the -threaded flag,
hWaitForInput t where t >= 0 will block all other Haskell
threads for the duration of the call. It behaves like a
safe foreign call in this respect.

Computation hGetContentshdl returns the list of characters
corresponding to the unread portion of the channel or file managed
by hdl, which is put into an intermediate state, semi-closed.
In this state, hdl is effectively closed,
but items are read from hdl on demand and accumulated in a special
list returned by hGetContentshdl.

Any operation that fails because a handle is closed,
also fails if a handle is semi-closed. The only exception is hClose.
A semi-closed handle becomes closed:

if hClose is applied to it;

if an I/O error occurs when reading an item from the handle;

or once the entire contents of the handle has been read.

Once a semi-closed handle becomes closed, the contents of the
associated list becomes fixed. The contents of this final list is
only partially specified: it will contain at least all the items of
the stream that were evaluated prior to the handle becoming closed.

Any I/O errors encountered while a handle is semi-closed are simply
discarded.

hGetBufhdl buf count reads data from the handle hdl
into the buffer buf until either EOF is reached or
count 8-bit bytes have been read.
It returns the number of bytes actually read. This may be zero if
EOF was reached before any data was read (or if count is zero).

hGetBuf ignores whatever TextEncoding the Handle is currently
using, and reads bytes directly from the underlying IO device.

hGetBuf never raises an EOF exception, instead it returns a value
smaller than count.

If the handle is a pipe or socket, and the writing end
is closed, hGetBuf will behave as if EOF was reached.

hGetBufNonBlockinghdl buf count reads data from the handle hdl
into the buffer buf until either EOF is reached, or
count 8-bit bytes have been read, or there is no more data available
to read immediately.

ResourceVanished if the handle is a pipe or socket, and the
reading end is closed. (If this is a POSIX system, and the program
has not asked to ignore SIGPIPE, then a SIGPIPE may be delivered
instead, whose default action is to terminate the program).